PROJECT SUMMARY/ABSTRACT Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide. The approximately 600,000 patients diagnosed with HNSCC each year face a five-year survival rate of only 50%, a statistic that has not improved significantly in decades. Despite the increasing use of targeted therapies in cancer treatment, cetuximab (Erbitux), a monoclonal antibody that inhibits the epidermal growth factor receptor (EGFR), is one of only three targeted agents that are FDA approved for treatment of HNSCC. Resistance to cetuximab-containing therapy remains a major obstacle; thus, identification and targeting of mediators of cetuximab resistance is needed to improve patient outcomes. Although a number of mechanisms of resistance to cetuximab have been identified in preclinical studies, these efforts have not yet resulted in a co-targeting strategy that has proven effective in overcoming cetuximab resistance in the clinic. Cetuximab exerts its antitumor effects by inhibiting EGFR-mediated activation of tumor cell proliferation and survival pathways and by inducing antibody-dependent cell-mediated cytotoxicity (ADCC), in which natural killer (NK) cells, a type of innate immune cell, induce lysis of tumor cells coated with cetuximab. One potential mediator of cetuximab resistance is interleukin 6 (IL-6), a cytokine that promotes tumor cell proliferation and survival and has a number of immunosuppressive functions in the tumor microenvironment. Our preliminary data suggest that expression of IL-6 is increased in cetuximab-resistant HNSCC cells and that these cells are sensitive to inhibition of the IL-6 pathway. Based on this and other preclinical and clinical evidence supporting a role for IL-6 in cetuximab resistance, we hypothesize that IL-6 mediates resistance to cetuximab both by offsetting the effects of EGFR inhibition in tumor cells and by impeding the ability of NK cells to mediate lysis of cetuximab- opsonized tumor cells. To test this hypothesis, I will assess the antitumor efficacy of combining EGFR- and IL-6 pathway-targeted therapies in preclinical models of HNSCC. I will also analyze the effects of IL-6 on cetuximab-induced ADCC and determine whether inhibition of IL-6 signaling can enhance the ability of NK cells to mediate cetuximab-induced ADCC. These studies will elucidate the role of IL-6 in cetuximab resistance and may provide support for the use of inhibitors of the IL-6 pathway in combination with cetuximab as a therapeutic strategy for HNSCC.